Metal hydrides can be used to store hydrogen gas. Metal hydrides can absorb gaseous hydrogen and chemically store it. Thus, a metal gas storage material, which may be in a crystalline form, can be hydrolyzed by absorbing hydrogen to form a metal hydride. The term “metal hydride” will be used to refer to this example gas storage material in its varying states of hydrolization. The reaction is reversible and thus the hydrogen can be released when required. Typical metal hydrides suitable for storage of hydrogen include sodium hydride, aluminium hydride or lithium hydride. For efficient absorption of hydrogen the metal hydride can be “activated” by repeatedly presenting gaseous hydrogen to the metal hydride at elevated pressure. Once activated, in which the hydride may adopt a crystalline form, hydrogen may be presented to the hydride at an elevated pressure such that it is absorbed by the hydride and chemically stored in its structure. Thus, metal hydrides and other gas storage materials can be used in gas storage vessels to store gas, such as hydrogen for use by fuel cells. The absorption of hydrogen by the metal hydride is a reversible reaction and therefore the hydrogen gas can be extracted from the gas storage vessels when required.
It is desirable to reuse the gas storage vessels once the gas stored therein has been used. Thus, an efficient system for filling the gas storage vessels with a gas would be advantageous. The absorption of a gas by a gas storage material needs to be carefully controlled to ensure that vessel contains a consistent quantity of gas.